It is known to produce bisphenol A by reacting an excess amount of phenol with acetone in the presence of an acid catalyst. As the method for obtaining high-purity bisphenol A from the above reaction product, a method is known in which the reaction product is subjected to a crystallization treatment to precipitate the crystals of an adduct of bisphenol A and phenol (hereinafter referred to as “adduct crystals”), then a slurry of the obtained crystals is subjected to solid/liquid separation, and the phenol is removed from the recovered adduct crystals. In another method, bisphenol A crystals are precipitated instead of adduct crystals.
A continuous crystallization process is known as a method for purifying a large volume of crude material at high efficiency. In carrying out continuous crystallization, usually concentration of material is adjusted in the material preparation step, and the material is supplied to a crystallizer after the temperature has been adjusted to a degree not lower than the saturation temperature. In this case, an excess rise of temperature of the material leads to an increase of the quantity of the required heat in the crystallizer. This, in turn, duty of cooling the supersaturation, which not only induces an increased rate of nucleation to reduce the average size of crystals, but also encourages deposition of scale on the heat transferring surface of the cooler, the inner surfaces of the pipes, etc. Therefore, in the material-to-be-crystallized (hereinafter referred to as crystallization material) preparation step, usually the temperature is adjusted to a degree which is slightly (such as +3° C.) above the saturation temperature.
As a continuous crystallization process of adduct crystals, a method is known comprising the steps of crystallization, solid/liquid separation, washing and crystal re-dissolving with a plural number of (1st to (n-1)st) stages, in which (a) purified phenol is used as the washing liquid for the crystals obtained at the n-th stage, (b) the washings or mother liquor recovered at the n-th stage is used as the phenol solution for re-dissolving the crystals obtained at the n-1th stage, (c) the mother liquor or washings recovered at the n-th stage is used as the washing liquid for the crystals obtained at the n-1th stage, and (d) the concentration of impurities in the phenol solution for re-dissolving the crystals and the concentration of impurities in the washing liquid of the crystal at each of the 1st to n-1th stages are made higher than those at the succeeding stage (Japanese Patent Application Laid-Open (KOKAI) No. 5-117194).
However, according to the studies by the present inventors, it was found that the proposal in the above-mentioned patent to use the mother liquor exclusively for the dissolution of crystals as a way of effective utilization of the mother liquor in the crystallization process has the problem that the crystals tend to precipitate on the heat transferring surface of the cooler, the inner surfaces of the pipes, etc., to form a deposit called scale, jeopardizing the long-time stable operation of the apparatus. Particularly in the case of a crystallization apparatus having an external circulation type cooling system, the scale tends to deposit on the heat transferring surface of the cooler, the inner surfaces of the pipes located downstream of the meeting point of the crystallization material and the coolant slurry, and other parts to make it difficult or even unable to carry on the continuous long-time stable operation of the apparatus.
The present invention has been made in view of the above circumstances, and its object is to provide an improved continuous crystallization process which can be applied to continuous formation of adduct crystals or bisphenol A crystals, according to which the heat duty that needs to be removed in the crystallization operation is lessened and the solid deposition on the inner surfaces of the crystallizer, especially on the heat transferring surface of the cooler, is restrained to allow prolongation of the continuous operation period of the crystallizer.